1. Field of the Invention
The invention relates generally to the removal of metal from an acidic solution. More particularly, the invention relates to the use of a metal binding compound to bind a metal and a sorbent to sorb the compound and allow the compound and the metal bound thereto to be removed from the acidic solution.
2. Background of the Invention
The removal of metal from a variety of metal containing solutions, such as acid (rock) mine drainage and industrial wastewaters, is important for many reasons including to comply with environmental regulations. Examples of metal containing industrial wastewaters include those generated by leather tanning operations, textile manufacturing, electronic “chip” manufacturing, metal plating facilities, precious and semiprecious metal mining operations, nuclear fuel and nuclear weapons processing, and electric power generation (both nuclear and coal). Currently, there are several prior art approaches for removing metals from solution, although none of these approaches provides a perfect approach for all solutions, and new and useful systems and methods for removing metals from solution are desired and needed.
Acid mine drainage and ore processing water are exemplary metal ion bearing solutions. Acid mine drainage is a byproduct of hard rock mining operations that expose sulfide containing minerals to water and oxygen, inducing sulfide mineral oxidation. In aqueous systems, the oxidation of pyrite and other sulfide bearing minerals involves several oxidation/reduction reactions, some involving microbial catalysis, which can result in the formation of highly acidic conditions. As a result, active and abandoned hard rock mines generate millions of gallons of acidic runoff each year, which is commonly termed acid mine drainage. In the state of Colorado alone, it is estimated that there may be around 1000 or more miles of metal ion bearing streams that are impacted by acid mine drainage. Acid mine drainage is generally characterized by very low pH, elevated concentrations of dissolved iron Fe (II) and sulfate SO4−2, and depending on the local geology, oxidation/reduction conditions and pH, a suite of other dissolved metal cations and complexes. Exemplary metal cations found in many environmental waters include those of zinc, copper, mercury, lead, gold, silver, cadmium, uranium, chromium, and others. Many of these solutions are hazardous to humans, plants, and wildlife and have been mandated for treatment by regulatory agencies such as the Environmental Protection Agency (EPA). For example, in the state of Colorado several superfund sites have been dedicated to the prevention, containment and treatment of acid mine drainage and other mining associated metal containing solutions.
Among the remediation strategies that have been used to treat or otherwise recover metals from solution are controlled precipitation, membrane separation processes and immobilization on ion exchange resins. Controlled precipitation is generally accomplished by adding sufficient amounts of base (e.g., carbonate addition) to a metal containing water, processing water or wastewater, in order to shift chemical conditions to a point where metals have decreased solubility and precipitate as solids. One of the disadvantages of this approach is that the alkalinity additions used to drive the reliable precipitation of metals typically found in acid mine drainage, or many other metal containing wastewaters, are well in excess of natural levels and the corresponding reagent masses and volumes can be costly. In general, precipitation processes generate large amounts of metal laden sludge that is difficult and costly to move from the site (e.g., collect and transport) and otherwise dispose of, due in part to its amorphous structure and residual water content. Precious metal recovery from these types of sludges is generally not cost effective.
Ion exchange resins including zeolites have also been used to remove metals from solution. This generally involves introducing a metal containing water or wastewater through a resin bed, often configured as a packed column, to immobilize metal ions on/in sphereoidal beads, which are comprised of, or include, the active resin or zeolite. In this case, metals are exchanged on a charge equivalent basis for nonmetal species, which are liberated into solution as the metals are sequestered from solution. Among the disadvantages of this approach is that resin performance is very pH sensitive, results in other ions in solution (often hydrogen, an alkali (e.g. sodium) or alkali earth element) and that it is relatively expensive to implement; generally resin exchange is not well suited for in situ treatment (i.e., the waters need to be pumped through a packed bed rather than being treated in their natural place). Many, ion exchange resins that are employed for metal removal are so pH sensitive such that extremely narrow pH operating ranges are used to obtain effective exchange, which can need copious reagent additions prior to exchange treatment. Further, resin exchange processes are sensitive to the presence of suspended solids and colloidals such that pretreatments are often needed to remove particulate matter prior to ion exchange.
The prior art contains many examples of attempting to sorb metals directly from solution. While these processes are typically shown to be effective at neutral or higher pH, they are essentially ineffective in acidic pH ranges. See, for example:                Chen, T. N. and C. P. Haung (1992). “Treatment of zinc industrial; wastewater by combined chemical precipitation and activated carbon adsorption.” 24th Mid-atlantic industrial wastewater conference: 120-134.        Huang, C. P. and D. W. Blankenship (1984). “The removal of mercury(II) from dilute aqueous solution by activated carbon.” Wat. Res. 18(1): 37-46.        Kim, L. (1976). Adsorption of chromium on activated carbon. Gainesville, Fla., University of Florida.        Netzer, A. and D. E. Hughes (1984). “Adsorption of copper, lead and cobalt by activated carbon.” Wat. Res. 18(8): 927-933.        Reed, B. E. (1995). “Identification of removal mechanisms for lead in granular activated carbon(GAC) column.” Separation Science and Technology 30(1): 101-116.        SenGupta, A. K. (2002). Environmental separation of heavy metals. Boca Raton, Fla., CRC press LLC.        Smith, S. B. (1973). Trace metal removal by activated carbon. Traces of heavy metals in water:Removal Processes and monitoring, Princeton University, NJ.        Wilezak, A. and T. M. Keinath (1993). “Kinetics of sorption and desorption of copper(II) and lead(II) on activated carbon.” Water Environment Research 65(3): 238-244.        
The prior art also contains examples of attempts to use metal-coordinating organic compounds to enhance metal immobilization on activated carbon. These previous attempts used metal binding agents such as, for example: Ethylenediaminetetracetate (EDTA), porphyrin and porphyrin-containing compounds, citrate and citrate-containing compounds and dimercaprol. These attempts are outlined in the following scientific literature:                Reed, B. E. and S. K. Nonavinakere (1992). “Metal adsorption by activated carbon: Effect of complexing ligands, competing adsorbates, ionic strength, and background electrolyte.” Separation Science and Technology 27(14): 1985-2000.        Rubin, A. J. and D. L. Mercer (1987). “Effect of complexation on the sorption of cadmium by activated carbon.” Separation Science and Technology 22(5): 1359-1381.        Shay, M. and J. E. Etzel (1992). “Treatment of metal-containing wastewater by carbon adsorption of metal-chelate complexes.” Proc. Purdue Ind. Waste conf. 46th: 563-569.        
A general review of using complexing metal ligands to enhance the sorption of heavy metals is presented in:                SenGupta, A. K. (2002). Environmental separation of heavy metals. Boca Raton, Fla., CRC press LLC.        
It is recognized by the present invention, as will be described in further detail below, that such use of metal-coordinating organic compounds in the prior art, as exemplified by the last four citations immediately above, appear to share two difficulties: (i) there is limited enhancement of the immobilization of metals, as compared with activated carbon used alone, and (ii) process efficiency drops markedly at depressed pH (i.e., these attempts fail to be significant below pH 4.5). It is noted that the cited prior art appears to be devoid of any use of Benzotriazole, any of its derivatives, or other Benzotriazole-containing compounds, or; Benzothiazoles, any of its derivatives, or other benzothiazole-containing compounds in remediation processes that are performed with acidic solutions.
In view of the foregoing, the prior art is replete with examples of successful wastewater treatment at approximately neutral or higher pH in performing metal contamination remediation, as well as for other purposes. As one example of the latter, benzotriazoles, as used, for instance, in aircraft deicing fluid, were demonstrated to be removable from wastewater in the scientific literature using a sorbent. See Anaerobic Digestion of Deicing Fluids: Interactions and Toxicity of Corrosion Inhibitors and Surfactants by C. Gruden, et al. Water Environment Research 74 (2): 149-158 (2002) and Fate and Toxicity of Aircraft Deicing Fluid Additives through Anaerobic Digestion by C. L. Gruden, et al. Water Environment Research. 73(1):72-79 (2001).
With such a catalog of successful prior art approaches available for use in basic wastewater, it is submitted that the prior art teaches directly away from attempting to remove metal contamination from acidic solutions. One of ordinary skill in the art, when faced with the task of removing metal contamination from an acidic solution is motivated, at the outset, by the prior art to first convert the acidic solution to a near neutral or basic solution for the subsequent application of these successful prior art approaches. Unfortunately, conversion to the basic solution can itself be quite costly, while introducing further contamination issues.
Aside from such general past use of metal-coordinating organic compounds, there have been attempts to remove metal contamination from acidic solutions, all of these attempts are considered to have met with limited, if any, success at least from a practical standpoint. For example, in the 1980's, a binder and sorbent were added to such an acidic solution. It was found that the binder, in this particular attempt, made essentially no contribution to a minimal amount of metal that was removed. That is, consistent with the remaining prior art, the result with the binder was no different than the result with the sorbent alone, clearly evidencing that the mechanism of metal removal was limited to sorption of the metal directly onto the sorbent. For these reasons, it is submitted there is simply no practical approach available in the prior art for treating metal contaminated acidic wastewater. This latter assertion may be evidenced by the ever increasing amount of untreated acidic wastewater contamination that is present throughout the world.
Accordingly, there is a need in the art for other useful and novel systems and methods for removing metals from solutions.